Bright rhodium electrodeposition

ABSTRACT

Disclosed is an electroplating solution for obtaining bright white rhodium electrodeposits, said solution containing rhodium in the form of a soluble sulfate or phosphate compound, together with an excess quantity of sulfuric acid, phosphoric acid, or mixtures of the two; the improvement of which is the addition of one or more nitrogen-containing heterocyclic organic compounds, in which at least one nitrogen is incorporated into a six-membered aromatic ring.

BACKGROUND OF THE INVENTION

Smooth surfaces of rhodium exhibit a specular reflectivity ofapproximately 80 percent throughout the visible spectral range,resulting in a brilliant white appearance. Of the precious metals, onlysilver exhibits a higher reflectivity over this spectral range; butsince silver is subject to tarnishing and rhodium is not, rhodium isordinarily the coating of choice, even as a thin overplate on silveritself.

In electroplating it is common practice to employ various additives,referred to as Brightening Agents, the effect of which is to reduce theaverage grain size of the electrodeposit. Ordinarily such grain sizereduction results in increased specular reflectivity, or brightness, ofthe deposit which in turn allows the deposit to be plated with highreflectivity to thicknesses greater than possible in the absence of thebrightening agent. It sometimes happens, however, that the increase inspecular reflectivity resulting from the use of a brightening agent isnot uniform across the spectral range of interest. In such a case, theperceived color of the electrodeposit will change, as well as thereflectivity. Since whiteness of appearance in rhodium electrodepositsis considered highly desirable, a suitable brightening agent for rhodiumwould be one which would increase the specular reflectivity of thedeposit while preserving its uniform spectral response in the visiblespectral range.

Additives for rhodium electroplating solutions have been discussed bySafranek (1). Only phenolsulfonic acid is cited as a brightening agent.This material had been investigated by Wiesner and Meers (2) whoreported its effective concentration range to be 1-1.5 milligrams pergram of rhodium in the plating solution (0.1-0.15 percent w/w). Inrhodium plating solutions containing 5.28 grams rhodium metal per litertogether with 33 milliliters concentrated sulfuric acid per liter,addition of 0.1 percent w/w of phenolsulfonic acid at 50° C. yieldsdeposits more reflective but somewhat darker in color than deposits fromthe same solution without additives. Addition of phenolsulfonic acid atratios of 0.25 percent w/w and over causes the deposit coloration tobecome progressively darker blue. Phenolsulfonic acid is not believed tohave ever been commercialized as a brightening agent for rhodium.

Lead and thallium in small (parts per million) quantities have been usedas whitening agents for rhodium electrodeposits. These do not increasethe specular reflectivity of the deposits, and they appear to beeffective only at low deposit thicknesses. Additionally, selenic acid,sulfamic acid, and magnesium sulfamate are used to control stress inrhodium electrodeposits. The mechanisms by which these operate are notwell understood. They do not appear to affect the reflectivity of thedeposits.

At this point there does not appear to exist a suitable brighteningagent for rhodium electrodeposits, i.e., one which is capable ofincreasing the specular reflectivity of the deposit while preserving orincreasing its perceived whiteness of color. It is an object of thisinvention to provide such a brightening agent or agents. It is a furtherobject of the invention to provide such a brightening agent or agentscapable of allowing rhodium electrodeposits of improved specularreflectivity and perceived whiteness of color to be plated to increasedthicknesses as are required for engineering as well as for decorativeapplications. It is yet a further object that the brightening agent oragents thus provided be capable of operating in all of theelectroplating systems commonly employed for rhodium; namely, thephosphate, sulfate, or mixed phosphate-sulfate systems.

SUMMARY OF THE INVENTION

This invention relates to aqueous electroplating solutions for thedeposition of rhodium and to the use thereof, in which solutions rhodiumis contained in the form of a soluble compound of rhodium with phosphateor sulfate; said solutions also containing an excess of phosphoric acid,sulfuric acid, or mixtures thereof; said solutions also containing anitrogen-bearing heterocylic organic compound at least one nitrogen ofwhich is incorporated into a six-membered aromatic ring.

The nitrogen-bearing heterocyclic organic compounds of this inventioninclude or are derived from pyridine, picoline, pyrimidine, pyridazine,or pyrazine. Compounds of this class brighten rhodium electrodeposits atthreshold concentration ratios as low as 0.25 milligram per gram ofrhodium (0.025 percent w/w). Typically, maximum whiteness of depositappearance is maintained up to concentration ratios about three to fivetimes the threshold level, after which the deposit color graduallydarkens, beginning at the lowest current densities. Brightness of thedeposit is maintained to concentration ratios over 50 times thethreshold value. As might be expected, various members of the classproduce somewhat different degrees of deposit brightness and whiteness.In all cases tested, however, the basic pattern demonstrated has beenthe same.

DETAILED DESCRIPTION OF THE INVENTION

As described above, the present invention is directed to anelectroplating solution for obtaining bright white rhodiumelectrodeposits. In preferred embodiments, the solution comprisesrhodium in the form of a soluble sulfate or phosphate compound, togetherwith an excess quantity of sulfuric acid, phosphoric acid, or mixturesof the two, and the improvement over known solutions of this typecomprises the addition of one or more nitrogen-containing heterocyclicorganic compounds, at least one nitrogen of which is incorporated into asix-membered aromatic ring. Particularly preferred nitrogen-containingheterocyclic compounds include the following:

A. Pyridine and derivatives thereof;

B. Nicotinic acid and derivatives thereof;

C. Isonicotinic acid and derivatives thereof;

D. Nicotinamide and derivatives thereof;

E. Pyridine 3-sulfonic acid and derivatives thereof;

F. 3-Pyridylacrylic acid and derivatives thereof;

G. 2-Aminopyridine and derivatives thereof;

H. 3-Aminopyridine and derivatives thereof;

I. Picoline and derivatives thereof;

J. Picolinic acid and derivatives thereof;

K. Pyrimidine and derivatives thereof;

L. 2-Aminopyrimidine and derivatives thereof;

M. Pyridazine and derivatives thereof;

N. 3,6-Dihydroxypyridazine and derivatives thereof;

O. Pyrazine and derivatives thereof;

P. Pyrazinamide and derivatives thereof;

As used herein, the terms “derivatives thereof” are defined as simplederivatives of the basic structures, particularly as illustrated above.These derivatives replace one or more hydrogen atoms with another groupselected from at least the following; halogen (Br, Cl, I, F), amino,nitro, hydroxy, hydroxy-C₁-C₆ alkyl, methoxy, cyano, benzyloxy, carboxy,benzoyl, N-oxide, mercapto, thiobenzyl, vinyl, phenylethyl, thio, andthe like.

The following examples describe various preferred embodiments of thepresent invention.

EXAMPLE 1

Sufficient water was used to form one liter of a solution containing5.28 grams of rhodium metal in the form of rhodium sulfate, togetherwith 33 milliliters of concentrated sulfuric acid. A 267 ml aliquot ofthis solution was plated in a Hull cell at 0.5 ampere for 5 minutes at50° C. using moving-vane agitation. The resulting deposit was white andbright up to an indicated current density of about 7.5 amperes persquare foot, above which the deposit was hazy and yellowish inappearance up to the upper current density edge.

EXAMPLE 2

A solution was made up as in Example 1 except additionally containing2.64 milligrams of pyridine. A 267 ml aliquot of this solution wasplated in a Hull cell under conditions identical to those described inExample 1. The resulting deposit was white and mirror-bright across theentire panel.

EXAMPLE 3

A solution was made up as in Example 1 except additionally containing13.2 milligrams of pyridine 3-sulfonic acid. A 267 ml aliquot of thissolution was plated in a Hull cell under conditions identical to thosedescribed in Example 1. The resulting deposit was white andmirror-bright across the entire panel.

EXAMPLE 4

A solution was made up as in Example 1 except additionally containing2.64 milligrams of nicotinamide. A 267 ml aliquot of this solution wasplated in a Hull cell under conditions identical to those described inExample 1. The resulting deposit was white and mirror-bright across theentire panel.

EXAMPLE 5

A solution was made up as in Example 1 except additionally containing13.2 milligrams of 2-picoline. A 267 ml aliquot of this solution wasplated in a Hull cell under conditions identical to those described inExample 1. The resulting deposit was white and mirror-bright across theentire panel.

EXAMPLE 6

A solution was made up as in Example 1 except additionally containing13.2 milligrams of pyrimidine. A 267 ml aliquot of this solution wasplated in a Hull cell under conditions identical to those described inExample 1. The resulting deposit was white and mirror-bright across theentire panel.

EXAMPLE 7

A solution was made up as in Example 1 except additionally containing2.64 milligrams of pyridazine. A 267 ml aliquot of this solution wasplated in a Hull cell under conditions identical to those described inExample 1. The resulting deposit was white and mirror-bright across theentire panel.

EXAMPLE 8

A solution was made up as in Example 1 except additionally containing2.64 milligrams of pyrazine. A 267 ml aliquot of this solution wasplated in a Hull cell under conditions identical to those described inExample 1. The resulting deposit was white and mirror-bright across theentire panel.

EXAMPLE 9

A solution was made up as in Example 1 except containing 10 gramsrhodium per liter, rather than 5.28 grams per liter, in the form ofrhodium sulfate. A gold-struck brass coupon of area 12.9 cm2 was platedat 10 mA/cm2 for two hours at 50° C. in this solution in a 1 literbeaker with spinbar agitation. The deposit weight was 0.3048 grams,indicating an approximate thickness of 19 micrometers. The depositappearance was matte grey.

EXAMPLE 10

The experiment of Example 9 was repeated using a solution to which hadbeen added 0.13 gram pyridine 3-sulfonic acid per liter. The depositweight obtained was 0.2138 grams, indicating an approximate thickness of13 micrometers. The deposit appearance was bright white.

EXAMPLE 11

Sufficient water was used to form one liter of a solution containing2.64 grams of rhodium metal in the form of rhodium phosphate, togetherwith 40 milliliters or concentrated phosphoric acid. A 267 ml aliquot ofthis solution was plated in a Hull cell at 0.5 ampere for 5 minutes at50° C. using moving-vane agitation. The resulting deposit was reflectivedark grey below about 3 amperes per square foot indicated, and hazylighter gray at current densities above 3 amperes per square foot.

EXAMPLE 12

A solution was made up as in Example 11 except additionally containing26.4 milligrams of pyridine 3-sulfonic acid. A 267 ml aliquot of thissolution was plated in a Hull cell under conditions identical to thosedescribed in Example 11. The deposit obtained was white andmirror-bright across the entire panel.

EXAMPLE 13

Sufficient water was used to form one liter of a solution containing2.11 grams rhodium metal in the form of rhodium sulfate, together with13.2 milliliters of concentrated sulfuric acid and also 26.4 millilitersof concentrated phosphoric acid. A 267 ml aliquot of this solution wasplated in a Hull cell at 0.5 ampere for 5 minutes at 50° C., usingmoving-vane agitation. The resulting deposit was white and bright belowabout 3 amperes per square foot indicated, and hazy and yellowish inappearance from about 3 amperes per square foot indicated up to theupper current density edge.

EXAMPLE 14

A solution was made up as in Example 13 except additionally containing2.64 milligrams of pyrazinamide. A 267 ml aliquot of this solution wasplated in a Hull cell at 0.5 ampere for 5 minutes at 50° C. usingmoving-vane agitation. The resulting deposit was white and mirror-brightacross the entire panel.

It will be understood by those skilled in the art that the Examplescited herein are illustrative of the invention, but that they do notrepresent the totality of the useful embodiments thereof. Not allderivatives of the nitrogen-containing heterocyclic organic compounds ofthis invention are readily available, but they can be prepared usingwell known synthetic methods. For one commercial source for a number ofsuch compounds see, http://www.flint.com.cn/products/Pyridine.htm. Allof the derivatives tested herein, however, have been effective for thepurposes of this invention.

REFERENCES

-   (1) W. H. Safranek, The Properties of Electrodeposited Metals and    Alloys, 2nd Ed., American Electroplaters and Surface Finishers    Society, Orlando, Fla., 1986, pp. 396-407-   (2) H. J. Wiesner and H. A. Meers, Plating, 43 (3) 347-356 (1956)

1. An electroplating solution for the deposition of rhodium; saidsolution containing rhodium in the form of a soluble sulfate orphosphate, together with an excess of sulfuric acid, phosphoric acid, ormixtures of the two; the improvement of which comprises the addition ofone or more nitrogen-containing heterocyclic organic compounds, in whichat least one nitrogen is incorporated into a six-membered aromatic ring.2. The electroplating solution of claim 1, wherein rhodium is in theform of rhodium sulfate.
 3. The electroplating solution of claim 1,wherein the rhodium is in the form of rhodium phosphate.
 4. Theelectroplating solution of claims 1, 2 or 3, wherein the electrolyte issulfuric acid.
 5. The electroplating solution of claims 1, 2 or 3,wherein the electrolyte is phosphoric acid.
 6. The electroplatingsolution of claims 1, 2 or 3, wherein the electrolyte is a mixture ofsulfuric and phosphoric acids.
 7. The electroplating solution of claims1, 2 or 3, wherein the nitrogen-containing heterocyclic organic compoundis pyridine or a derivative thereof.
 8. The electroplating solution ofclaim 7, wherein the nitrogen containing heterocyclic organic compoundis nicotinic acid.
 9. The electroplating solution of claim 7, whereinthe nitrogen containing heterocyclic organic compound is isonicotinicacid.
 10. The electroplating solution of claim 7, wherein the nitrogencontaining heterocyclic organic compound is nicotinamide.
 11. Theelectroplating solution of claim 7, wherein the nitrogen containingheterocyclic organic compound is pyridine 3-sulfonic acid.
 12. Theelectroplating solution of claim 7, wherein the nitrogen containingheterocyclic organic compound is 3-pyridylacrylic acid.
 13. Theelectroplating solution of claim 7, wherein the nitrogen containingheterocyclic organic compound is 2-aminopyridine.
 14. The electroplatingsolution of claim 7, wherein the nitrogen containing heterocyclicorganic compound is 3-aminopyridine.
 15. The electroplating solution ofclaim 7, wherein the nitrogen containing heterocyclic organic compoundis picoline.
 16. The electroplating solution of claim 7, wherein thenitrogen containing heterocyclic organic compound is picolinic acid. 17.The electroplating solution of claims 1, 2 or 3, wherein thenitrogen-containing heterocyclic organic compound is pyrimidine or aderivative thereof.
 18. The electroplating solution of claim 17, whereinthe nitrogen containing heterocyclic organic compound is2-aminopyrimidine.
 19. The electroplating solution of claims 1, 2 or 3,wherein the nitrogen-containing heterocyclic organic compound ispyridazine or a derivative thereof.
 20. The electroplating solution ofclaim 19, wherein the nitrogen containing heterocyclic organic compoundis 3,6-dihydroxypyridazine.
 21. The electroplating solution of claims 1,2 or 3, wherein the nitrogen-containing heterocyclic organic compound ispyrazine or a derivative thereof.
 22. The electroplating solution ofclaim 21, wherein the nitrogen containing heterocyclic organic compoundis pyrazinamide.
 23. An electroplating process comprising the steps of:(a) immersing a substrate in an electroplating solution containingrhodium in the form of a soluble sulfate or phosphate, together with anexcess of sulfuric acid, phosphoric acid, or mixtures of the two;wherein the solution further comprises one or more nitrogen-containingheterocyclic organic compounds, in which at least one nitrogen isincorporated into a six-membered aromatic ring; and (b) applying acathodic current to the substrate for the purpose of obtaining a brightand white deposit of rhodium thereon.
 24. The electroplating process ofclaim 23, wherein rhodium is in the form of rhodium sulfate.
 25. Theelectroplating process of claim 23, wherein the rhodium is in the formof rhodium phosphate.
 26. The electroplating process of claims 23, 24 or25, wherein the electrolyte is sulfuric acid.
 27. The electroplatingprocess of claims 23, 24 or 25, wherein the electrolyte is phosphoricacid.
 28. The electroplating process of claims 23, 24 or 25, wherein theelectrolyte is a mixture of sulfuric and phosphoric acids.
 29. Theelectroplating process of claims 23, 24 or 25, wherein thenitrogen-containing heterocyclic organic compound is pyridine or aderivative thereof.
 30. The electroplating process of claim 29, whereinthe nitrogen containing heterocyclic organic compound is nicotinic acid.31. The electroplating process of claim 29, wherein the nitrogencontaining heterocyclic organic compound is isonicotinic acid.
 32. Theelectroplating process of claim 29, wherein the nitrogen containingheterocyclic organic compound is nicotinamide.
 33. The electroplatingprocess of claim 29, wherein the nitrogen containing heterocyclicorganic compound is pyridine 3-sulfonic acid.
 34. The electroplatingprocess of claim 29, wherein the nitrogen containing heterocyclicorganic compound is 3-pyridylacrylic acid.
 35. The electroplatingprocess of claim 29, wherein the nitrogen containing heterocyclicorganic compound is 2-aminopyridine.
 36. The electroplating process ofclaim 29, wherein the nitrogen containing heterocyclic organic compoundis 3-aminopyridine.
 37. The electroplating process of claim 29, whereinthe nitrogen containing heterocyclic organic compound is picoline. 38.The electroplating process of claim 29, wherein the nitrogen containingheterocyclic organic compound is picolinic acid.
 39. The electroplatingprocess of claims 23, 24 or 25, wherein the nitrogen-containingheterocyclic organic compound is pyrimidine or a derivative thereof. 40.The electroplating process of claim 39, wherein the nitrogen containingheterocyclic organic compound is 2-aminopyrimidine.
 41. Theelectroplating process of claims 23, 24 or 25, wherein thenitrogen-containing heterocyclic organic compound is pyridazine or aderivative thereof.
 42. The electroplating process of claim 41, whereinthe nitrogen containing heterocyclic organic compound is3,6-dihydroxypyridazine.
 43. The electroplating process of claims 23, 24or 25, wherein the nitrogen-containing heterocyclic organic compound ispyrazine or a derivative thereof.
 44. The electroplating process ofclaim 43, wherein the nitrogen containing heterocyclic organic compoundis pyrazinamide.